ESR1 and breast cancer: The breast cancer was classified into four subtypes: hormone receptor (HR) +/ human epidermal growth factor receptor 2 (HER2) − (i.e., estrogen receptor [ER] + and/or progesterone receptor [PR] +, HER2−); HR+/HER2+ (ER+ and/or PR+, HER2+), HR−/HER2+ (ER− and PR−, and HER2+); and HR−/HER2− (ER− and PR−, and HER2−).